1. Field of the Invention
The present invention relates to a photosemiconductor device, a method for fabricating the photosemiconductor device and a method for driving the photosemiconductor device, more specifically a photosemicondcutor device which has a wide wavelength variation range and can provide high photooutputs, a method for driving the photosemiconductor device, and a photosemiconductor device including a tunable twin guide laser and an optical waveguide integrated on one and the same substrate and a method for fabricating the photosemiconductor device.
2. Description of the Related Art
Presently, in the trunk transmission systems of large-capacity optical communication networks, WDM (Wavelength Division Multiplexing) method, which multiplexes photosignals along a wavelength axes for larger transmission capacities, is used. In the WDM, a number of semiconductor lasers must be larger for a larger multiplexing number. The same number or more back-up light sources are also required. The large number of the parts complicates the inventory control.
In such background, it is expected to simplify the inventory control by using a wavelength selective light source which can vary the oscillation wavelength. The wavelength selective light source used in the WDM is required to have a wide continuous wavelength variation width.
As the wavelength selective light source, various wavelength selective lasers have been so far proposed. To give examples, the wavelength selective laser of the type that a DFB laser or a DBR laser is used to control the temperature to thereby vary the oscillation wavelength, and the wavelength selective laser of the type that a DBR laser is used to vary the oscillation wavelength by controlling the value of the current to be supplied to the tuning part thereof are known. As the DBR type are also known GCSR-DBR lasers having the filter function, and SG/SSG-DBR lasers which vary the oscillation wavelength by the modulation by partial diffraction grating patterns. However, these laser light sources have disadvantages such that they cannot provide larger photooutputs when they vary the wavelength, the range in which the wavelength can be continuously varied is as narrow as some nanometers, which makes the wavelength control complicated, and the mode hopping causes the discontinuous wavelength variation, etc.
Among these wavelength selective light sources, tunable twin guide DFB laser (hereafter called TTG-DFB laser) is characterized by a continuous wavelength variation width of about 8 nm, which is relatively wide, and by the simple wavelength variation method. Tunable twin guide lasers (hereinafter called TTG-LD), such as the TTG-DFB laser, etc. are described in, e.g., Japanese Patent Application Unexamined Publication No. Hei 07-131121 (1995), Japanese Patent Application Unexamined Publication No. Hei 07-326820 (1995) and Specification of U.S. Pat. No. 5,048,049 (1991).
The TTG-LD, which is known one laser which can control the above-described oscillation wavelength, has advantages that it can continuously control the oscillation wavelength in a single mode and can perform the wavelength control at high speed (refer to, e.g., Specification of U.S. Pat. No. 5,048,049). Furthermore, the TTG-LD also has the advantage that the wavelength control mechanism is simple. Owing to the advantages, the TTG-LD including the TTG-DFB laser is expected to be applicable to the light source, etc. for the optical communication of the WDM.
The conventional TTG-LD disclosed in Specification of U.S. Pat. No. 5,048,049 will be explained with reference to FIG. 41. FIG. 41 is a sectional view of the conventional TTG-LD, which shows a structure thereof.
A p type InP buffer layer 502 is formed on a p type InP semiconductor substrate 500. A p type electrode 504 for the wavelength control is formed on the underside of the semiconductor substrate 500.
On the buffer layer 502, a mesa stripe of InGaAsP wavelength control layer 506, an n type InP intermediate layer 508, an InGaAsP MQW active layer 510 and a p type InP clad layer 512 are formed by being laid one on another in the stated order and etched. A quaternary diffraction grating layer 514 having a diffraction grating formed in is formed between the buffer layer 502 and the wavelength tuning layer 506.
On the buffer layer 502 on both sides of the mesa stripe, an n type InP buried layer 516 is formed and buries the mesa stripe.
A p type InP cap layer 518 is formed on the buried layer 516 and the clad layer 512 of the mesa stripe. On the cap layer 518 a p type electrode 520 is formed electrically connected to the MQW layer 510 via the cap layer 518 and the clad layer 512.
On the buried layer 516 an n type electrode 522 is formed electrically connected to the intermediate layer 508 via the buried layer 516.
In the TTG-LD having the above-described structure, the p type electrode 504 formed on the underside of the semiconductor substrate 500 injects current into the wavelength tuning layer 506 formed below the intermediate layer 508 via the semiconductor substrate 500 and the buffer layer 502. On the other hand, the p type electrode 520 formed on the cap layer 518 injects current into the MQW layer 510 formed on the upper side of the intermediate layer 508 via the cap layer 518 and the clad layer 512.
The intermediate layer 508 is formed between the wavelength tuning layer 506 and the MQW layer 510 and is connected to the outside earth potential by the n type electrode 522. That is, the intermediate layer 508 connected to the earth potential functions as the common earth potential of the elements. Thus, the intermediate layer 508 connected to the outside earth potential makes the two functional layers, i.e., the MQW active layer 510 and the wavelength tuning layer 506 electrically independent of each other. Accordingly, the TTG-LD having this structure controls the current amount injected into the respective functional layers to thereby perform the laser oscillation control and the oscillation wavelength control independent of each other.
As described above, the TTG-DFB laser is characterized by the continuous, relatively wide wavelength variation range and the easy wavelength variation control method, and is attractive in comparison with the other lasers. However, the TTG-DFB laser as well as the other lasers has the disadvantage that the injection of current in the wavelength tuning layer for varying a wavelength to a shorter wavelength increases the internal loss of the laser, causing large decrease of photooutput.
Means for compensating the photooutput decrease is further injection of current into the active layer. However, this causes, on the other hand, the active layer temperature, i.e., the element temperature increase, causing the opposite effect of shifting the oscillation wavelength to a longer wavelength. Resultantly, the wavelength variation width is decreased. Furthermore, the wavelength shift due to the temperature increase must be again controlled by the wavelength tuning current, which complicates the wavelength variation method.
Japanese Patent Application Unexamined Publication No. Hei 07-326820 (1995) discloses a photosemiconductor device comprising a TTG-DFB laser, a photo-phase adjuster, a photo-intensity adjuster, and a reflection mirror. Japanese Patent Application Unexamined Publication No. Hei 07-326820 (1995) discloses that the absorption loss in a TTG-DFB laser is compensated by adjusting return light from the photo-phase adjuster and the photo-intensity adjuster. However, it cannot be said that the control disclosed in Japanese Patent Application Unexamined Publication No. Hei 07-326820 (1995) is easy. The integrated structure is not of optical elements having gains, and is not expected to much increase the photooutput.
As described above, the above-described photosemiconductor devices cannot realize high photooutputs although having wide wavelength variation ranges. Photosemiconductor devices which can simultaneously realize both have been expected.
The TTG-LD includes, as described above, 2 layers, the MQW active layer for current to be injected into from the electrode formed on the upper surface of the substrate and the wavelength tuning layer for current to be injected into from the electrode formed on the underside of the substrate. Accordingly, when the TTG-LD is integrated with other elements, such as an optical waveguide, etc., on one and the same substrate, disadvantages, such as characteristic deterioration, etc., will take place.
The technique of integrating the TTG-LD with other elements, such as an optical waveguide, etc. on one and the same substrate without causing the characteristic deterioration will be essential to the wide applicability of the TTG-LD.